The concept of variable valve control is used and is being further developed with the aim of improving the thermal efficiency of internal combustion engines. The principle of function of this concept is based on a variation of the valve opening times and the valve lifts of the gas exchange valves, i. e. of the inlet and outlet valves. The basic principle is that larger valve lifts (high lift) and shorter opening times ‘enhance the performance of the engine and enable a higher peak performance of the internal combustion engine. At the same time, however, the fuel consumption also increases while the torque available is reduced. In contrast, a small valve lift improves, in particular, the torque at low engine speeds. Through a complete switching-off of one or more valves (deactivation of a cylinder), a significant saving of fuel can be achieved. Efforts are therefore directed to optimizing valve control with a view to a favorable development of torque arid power taking into account that fuel consumption and exhaust gas generation should be as low as possible.
Known variable valve controls are realized with switchable cam follower assemblies (rocker arm, finger lever or oscillating lever assemblies) in which a multi-cam camshaft varies the valve lift of the gas exchange valve or valves. For this purpose, a plurality of lever arms that are driven by cams of different shape (sharpness) and size are provided. The size of the cams changes the adjusting displacement, and the sharpness of the cams has an influence on the opening and closing times. By switching between the lever arms, different adjusting displacements, i.e. valve lifts and/or opening and closing times are imparted to the valve in question. Through the position of the cams and a purposed adjustment of the camshaft(s), an additional influence can be exercised on the point of time and duration of opening and closing of the individual inlet and outlet valves.
A valve train with switchable rocker arms is shown in U.S. Pat. No. 6,763,793 B2. This valve train comprises two rocker arms that are mounted for tilting on a first rocker arm shaft and are driven by differently shaped cams. A piston is mounted in a cylindrical space in one of the rocker arms for sliding crosswise to the axis of the rocker arm shaft. An engaging member configured on the piston can engage with an engaging projection on the other rocker arm. Switching between an engaged position and a separated position is achieved through a switching device comprising a means for a hydraulic loading of the piston and a readjusting spring arranged within the cylindrical space.
In the engaged position, both rocker arms tilt, and the cam associated to the second rocker arm, i.e. the cam of the rocker arm with the engaging projection produces a valve lift for high engine speeds of the activated inlet or outlet valve.
In the separated position, both rocker arms tilt independently of each other, and the second rocker arm (high speed) freewheels. The cam associated to the first rocker arm, i.e. the cam of the rocker arm with the piston, now produces a valve lift for low engine speeds of the activated inlet or outlet valve.
Comparable valve trains comprising a low speed lever and a high speed lever, with a piston installed crosswise in a lever shaft, through which the two levers can engage each other are likewise known from U.S. Pat. Nos. 5,186,128, 5,320,082, 5,370,090, 5,423,295 and 5,429,079.
A drawback of the prior art is that the construction of these cam follower assemblies, particularly the crosswise installation of the piston in the cylindrical space, is relatively complex. An additional requirement is an anti-rotation device that fixes the piston in an aligned position. Moreover, each cylinder requires a separate rocker arm shaft, so that the complexity and costs of installation and for a switching oil supply system increase. Besides this, each of these rocker arm shafts rotates together with the high speed lever concerned, so that the overall inertia of rotation of the valve train increases. Finally, a gas exchange valve operated through such an arrangement can only be switched between a high speed setting and a low speed setting. A valve deactivation of the gas exchange valve concerned necessitates additional measures.